A bionic arm: After undergoing a novel surgical procedure, Claudia Mitchell can control a prosthetic arm much as she once controlled her real arm, performing complex movements such as picking up small objects and dropping them into a cup.

People who have lost an arm have not traditionally had much hope of ever regaining meaningful function. Prosthetic arms have been controlled in a rudimentary way, by transforming residual shoulder movements or muscle signals into the ­simplest movement commands. These artificial arms cannot do two things at once, much less three or four. Amputees often toss them in the closet out of sheer frustration, somewhat stung by the fact that leg amputees have far better products available to them.

But the situation is starting to change, thanks to a team led by Todd Kuiken, director of the Rehabilitation Institute of Chicago’s Center for Bionic Medicine. Kuiken has developed a novel surgical technique that, when paired with both motorized prosthetic arms already on the market and experimental bionic arms developed through a Defense Advanced Research Projects Agency (DARPA) program, affords amputees a remarkable degree of dexterity. Claudia Mitchell, who lost her arm in a motorcycle wreck in 2004, remembers putting on a prosthesis after undergoing Kuiken’s procedure and seeing it work for the first time: “You could not wipe that grin off of my face. I can now iron a shirt again like nobody’s business.” Mitchell has become a hit at parties. “People can’t believe how this thing works,” she says. “They want to see me do things with it.”

The device is activated by commands from surviving arm nerves that have been transplanted and rewired to muscles elsewhere–typically, as in Mitchell’s case, in the chest. The nerves send electrical signals to control the prosthetic arm, with results so natural that observers often don’t realize the arm is bionic until they listen closely for the sound of whirring motors. Called targeted muscle reinnervation, the procedure is unique because it permits intuitive control over the robotic limb. After about six months of healing, patients can move the arm merely by thinking about what they want it to do, just as they once did with their real arms. Tell Mitchell “Bend your arm,” and the muscles in her chest flinch instantaneously–a most peculiar sight. But she is not thinking about moving her chest muscles. Rather, she is thinking about bending her arm, and that thought moves the chest muscles to make the robotic arm do her bidding.

Kuiken recently published promising test results in the Journal of the American Medical Association, showing that five patients told to perform 10 different arm movements with a virtual prosthesis could do so almost as readily as non-amputees in a control group: their response time was less than a quarter of a second longer. (The virtual prosthesis allows scientists to more easily figure out the speed and level of control that can be gleaned from muscle signals. Researchers performed similar experiments with mechanical arms.) In an accompanying editorial, the pioneering biomedical engineer Gerald Loeb wrote, “The speed as well as the accuracy of the movements represent substantial improvements over previous systems. Even more important, however, is the ease with which patients learned to perform tasks requiring coordinated motion in more than one joint.” He concluded, “With increasing functional capabilities, patients with upper-extremity amputations may derive exceptional benefit from prosthetic arms, just as legions of patients with lower-extremity amputations now lead remarkably normal and even athletic lives.” (Leg prostheses have been further along in development because there is a bigger market for them: 90 percent of amputees have lost lower limbs. Also, legs don’t require as much dexterity as arms.)